176 
DIFFICULTIES ON THEOBY. 
Chap. VI. 
the same rule will probably apply to both; and if we in 
imagination adapt a varying species to a very large area, 
we shall have to adapt two varieties to two large areas, 
and a third variety to a narrow intermediate zone. The 
intermediate variety, consequently, will exist in lesser 
numbers from inhabiting a narrow and lesser area; and 
practically, as far as I can make out, this rule holds good 
with varieties in a state of nature. I have met with 
striking instances of the rule in the case of varieties 
intermediate between well-marked varieties in the genus 
Balanus. And it would appear from information given 
me by Mr. Watson, Dr. Asa Gray, and Mr. Wollaston, 
that generally when varieties intermediate between 
two other forms occur, they are much rarer numerically 
than the forms which they connect. Now, if we may 
trust these facts and inferences, and therefore conclude 
that varieties linking two other varieties together have 
generally existed in lesser numbers than the forms which 
they connect, then, I think, we can understand why in¬ 
termediate varieties should not endure for very long 
periods;—why as a general rule they should be exter¬ 
minated and disappear, sooner than the forms which 
they originally linked together. 
For any form existing in lesser numbers would, as 
already remarked, run a greater chance of being exter¬ 
minated than one existing in large numbers; and in 
this particular case the intermediate form would be 
eminently liable to the inroads of closely allied forms 
existing on both sides of it. But a far more important 
consideration, as I believe, is that, during the process 
of further modification, by which two varieties are sup¬ 
posed on my theory to be converted and perfected into 
two distinct species, the two which exist in larger 
numbers from inhabiting larger areas, will have a great 
advantage over the intermediate variety, which exists 
